Current semiconductor radiation detectors employ semiconductor crystals that interact with ionizing radiation to produce electron-hole pairs in the semiconductor. A detector may then detect the electrons and holes electrically, e.g., by collecting charges at electrodes in a photoconductive detector, or from photons emitted when electrons and holes recombine, e.g., as in a scintillation counter. The energy of the ionizing radiation may be determined from the electrical or light signal thus produced because, in general, a particle of radiation interacting with a semiconductor creates a number of electrons-hole pairs that indicates the energy of the radiation particle.
The efficiency of charge carrier collection in photoconductive detectors generally depends on the “trapping length,” which is the mean distance traveled by a charge carrier in the semiconductor crystal. A semiconductor crystal particularly needs to provide a “trapping length” that is long enough that the charge carriers are likely to travel from their creation points to collecting electrodes. The trapping length depends on the lifetime of charge carriers in a semiconductor and may be expressed as a product of the carrier lifetime, the carrier mobility, and the electric field resulting from a bias voltage applied in the detector. Recombination and trapping of charge carriers limit the carrier lifetimes in a semiconductor and may be the most important factors limiting the efficiency for collection of the charge carriers.
The disparity of trapping lengths of electrons and holes in the semiconductor crystals further complicates efficient collection of charge carriers. In CdTe based semiconductors such as CdZnTe (CZT) and in other wide bandgap, high Z compound semiconductors such as HgI2, the mobility of holes is much lower than mobility of electrons. The slower movement of holes can slow detection of the holes and distort the shape of the electrical signal generated when a radiation particle interacts with the semiconductor crystal. The low drift velocity of holes in the semiconductors used in photoconductive detectors therefore has an adverse effect on the response and performance of the photoconductive detectors. A photoconductive detector can reduce the effect of the low drift velocity of holes by implementing a single-polarity charge sensing mode, which reduces the contribution of holes to the measurement signal. In a Coplanar Grid (CPG) detector, a single-polarity charging mode is implemented using electrodes formed as two grids and operated in a subtraction mode that subtracts collected hole charge from the collected electron charge. [See Benjamin W. Sturm, Zhong He, Edgar Rhodes, Thomas H. Zurbuchen, Patrick L. Koehn, Proc. of SPIE Vol. 5540 (SPIE, Bellingham, W A, 2004) and Glenn F. Knoll, Radiation Detection and Measurement (John Wiley & Sons, Inc., 4th ed., 2010.] An alternative solution to the problem of holes having the low mobility in a photoconductive detector is based on a planar detector configuration with a continuous cathode electrode and a pixelated anode electrode that improves energy resolution by minimizing sensitivity to the motion of the positive charges (holes) that may not be completely collected. [See Glenn F. Knoll, Radiation Detection and Measurement, John Wiley & Sons, Inc., 4th ed., 2010.] However, these detector arrangements have complex electrode configurations and complex signal management electronics that increase the fabrication cost of such detectors and reduce the reliability of the detectors.
The CPG and pixelated detector configurations for electrodes still do not resolve issues associated with trapping and recombination of charge carriers limiting carrier lifetime. Improving carrier lifetimes generally requires control of crystal growth processes so that the semiconductor crystals used in detectors have fewer defects or other features that reduce charge carrier lifetimes. Further, manufacture of semiconductor radiation detectors generally requires characterization and monitoring of the semiconductors properties that affect radiation detectors performance. For wide-bandgap semiconductors, a standard approach tests semiconductor crystals by measuring a product (commonly called the “mu-tau product”) of carrier mobility and lifetime. The goal of the testing is to identify semiconductor crystals having a “mu-tau product” high enough for use in semiconductor detectors. However, conventional measurements of the “mu-tau product” do not reflect the effects associated with the electrostatic trapping of minority carriers at the electrostatic potential barrier formed at or around extended defects. This leads to a poor correlation of the measured “mu-tau product” of the modern detector-grade crystals with the performance of semiconductor-based radiation detectors and specifically leads to poor correlation with the energy resolution of the radiation detectors.